Basically  those tools aren't reading the DOM but Google can which is why it can see your site's title tags, H1s etc. Your site is using client-side rendering which Google can crawl through. Notice how if you go to a given page and click "View Source", none of the page's content appears.

While it appears Google is reading the content in the pages I looked at, I would definitely look into this more to see if Google is able to crawl/index the content on all of your site's pages. Client side rendering is less reliable than SSR so there might be instances where Google isn't reading sections of your content.

No problem! A good golden rule of JavaScript SEO is to always SSR where possible. Let me know if you have any other questions!

In the HTML of your pages there's an <a href="">link with "javascript:void(0". It appears that Google is getting into those. Is possible, remove that link or take it out of an</a> <a href="">element. Otherwise, you should be OK, those pages should 404. </a>

Good question! I've seen this happen with a few clients.

Here is my process for reviewing 5xx errors:

1. Manually check a handful of reported 5xx URLs and note their status codes. Often times upon manual inspection, you'll often find that these pages return different status codes (200, 4xx).
2. Perform a crawl using Screaming Frog and watch the "Status Codes" report. Watch to see if URL go from getting crawled as 200 to 5xx status codes.
3. If this is the case, the issue might that your hosting can't handle the requests getting sent to your site. If not and your URLs are getting reported as 200, it might been a temporary status the crawler found.
4. If possible, check your log files to see if Googlebot is returning a large number of 5xx errors.

Overall, if you only find a few 5xx errors in Search Console and your log files but don't find them in when you crawl the site, you're probably OK. However, if a crawl consistently reveals them or you see a large number in your log files then it's a high priority item to flag to your developers. You may need to consider upgrading your hosting or rolling back any recent development changes that were made. Definitely lean on your developer's insights here.